1. Field of the Invention
The present invention relates in general to electrochromic (sometimes referred to herein as “EC”) compounds for use in solution phase electrochromic devices and, more particularly, to cathodic electrochromic compounds comprising substituted viologens (i.e. substituted 4,4′-bipyridinium compounds).
2. Background Art
Solution phase electrochromic devices have been known in the art for several years. See, for example, U.S. Pat. No. 4,902,108, entitled “SINGLE-COMPARTMENT, SELF-ERASING, SOLUTION-PHASE ELECTRO-CHROMIC DEVICES, SOLUTIONS FOR USE THEREIN, AND USES THEREOF,” which is hereby incorporated herein by reference in its entirety—including the references cited therein. In solution phase electrochromic devices, 4,4′-bipyridinium compounds, such as 1,1′-dioctyl-4,4′-bipyridinium bis(tetrafluoroborate), 1,1′-dibenzyl-2,2,6,6′-tetramethyl-4,4′-bipyridinium bis(tetrafluoroborate), 1,1′-dibenzyl-2,2′-dimethyl-4,4′-bipyridinium bis(tetrafluoroborate) and 1,1′-dibenzyl-4,4′-bipyridinium bis(tetrafluoroborate) are known to be used as cathodic materials. See, for example, U.S. Pat. No. 5,336,448, entitled “ELECTROCHROMIC DEVICES WITH BIPYRIDINIUM SALT SOLUTIONS,” which is hereby incorporated herein by reference in its entirety—including the references cited therein. While the utilization of solution phase electrochromic devices which incorporate 4,4′-bipyridinium compounds into their electrochromic mediums has become increasing popular among, for example, the automotive industry, the development of undesirable residual color within the electrochromic medium remains problematic. In fact, 1,1′-dibenzyl-4,4′-bipyridinium bis(tetrafluoroborate) which is void of β hydrogen atoms is known to exhibit poor thermal and/or UV stability.
Indeed, when a sufficient electrical potential difference is applied across the electrodes of a conventional electrochromic device (e.g. an EC window, mirror, aircraft transparency, display device, etcetera), the electrochromic medium becomes intentionally colored (i.e. a low transmission state) inasmuch as one or more of the anodic and the cathodic materials are oxidized and reduced, respectively. Specifically, the anodic materials are oxidized by donating electrons to the anode and the cathodic materials are reduced by accepting electrons from the cathode.
For most commercially available devices, when the electrical potential difference is removed or substantially diminished, the anodic and cathodic materials return to their native or unactivated state and, in turn, return the electrochromic medium to its colorless or nearly colorless state (i.e. a high transmission state). The application and removal of an electrical potential difference is conventionally known as a single cycle of the electrochromic device.
Scientists have observed that over a period of cycles and/or time, during normal operation of the electrochromic device, the electrochromic medium sometimes does not remain colorless in the high transmission state. In some instances, even in the absence of an electrical potential difference, undesirable coloration of the cathodic viologen compound is observed—likely due to an adverse chemical reaction and/or compound degradation.
It is therefore an object of the present invention, among others, to provide a cathodic compound for use in the medium of an electrochromic device that remedies the aforementioned detriments and/or complications associated with maintaining a colorless or nearly colorless electrochromic device while the device is in its high transmission state.
These and other objects of the present invention will become apparent in light of the present specification, claims, and drawings.